January Review # 6

1. Consider the following:


U an uncooked chicken

W a well-cooked chicken

B a burnt chicken


Which of the enthalpy versus reaction-progress diagrams shown to the left is correct?





2. Suppose that a certain reaction rate is defined by


R = k [W]3[Q]2


where k = reaction constant

[W] = concentration of gas W in moles/L

[Q] = concentration of gas Q in moles/L


A chemist repeated the reaction and found 40 mL of product after 140 s. Originally it had taken the reaction 70s to produce 40 mL. By what factor did he change the concentration of W if he had used half as much Q?




3. A helium balloon is attached to a string. The right mass is tied to the other end of the string so that the balloon is suspended in the air: it neither rises nor sinks. According to the principle of buoyancy, the mass needed to suspend the balloon is given by:


where mload = attached mass + mass of string and balloons plastic in grams

VB = volume of balloon

rair = density of air in g/L

rHe = density of He in g/L

volume of balloon = 10.0 L

temperature = 20.0 oC

pressure = 101.3 kPa

average molar mass of air = 28.9 g/mole

a. Calculate the mass needed to suspend the balloon.

b. Rewrite the formula so that a student can directly plug in V, P, R, T and the molar masses in order to obtain m.

c. Experimentally, how could you figure out the necessary mass using a bunch of grapes, a string and a He balloon? (I 've done this a few times when bored at weddings.)

d. BONUS: Derive the formula given in the problem.

4. Using the table below, find the average at which HI is produced in the last 20 seconds of the reaction. Express your answer in moles/s.

H2 + I2 2HI


Time (s)

Remaining Amount of Iodine (moles)

Amount of iodine that reacted(moles)














5. Given: O2(g) + H2(g) 2 OH(g) DH = 77.9 kJ

O2(g) 2 O(g) DH = 495 kJ

H2(g) 2 H(g) DH = 435.9 kJ


Calculate DH for the following reaction: O(g) + H(g) OH(g)


6. At constant temperature, what happens to the volume of an ideal gas if its

pressure is decreased to one third of its original value? Also show the result on a graph.


7. In each case, point out (with a simple yes/no) whether the activation energy will be raised in the main reaction.


a.                adding the preservative calcium propionate to bread to slow the growth of mold______

b.               treating a cut with iodine to inhibit the function of bacterial proteins._____

c.                destroying enzymes by adding mercury and silver_______

d.               producing chlorophyll(catalyst) in early spring_____

e.                adding lactase(an enzyme=catalyst) to milk to break down lactose_______